Infrared photometry of solar active regions

Simultaneous time series of broad-band images of two active regions close to the disk center were acquired at the maximum (0.80Μm) and minimum (1.55Μm) continuum opacities. Dark faculae are detected in images obtained as weighted intensity differences between both wave-length bands. The elements of quiet regions can be clearly distinguished from those of faculae and pores in scatter plots of brightness temperatures. There is a smooth transition between faculae and pores in the scatter plots. These facts are interpreted in terms of the balance between the inhibition of convective energy transport and the lateral radiative heating.     

Helicity of solar active regions from a dynamo model

We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. We use our solar dynamo model based on the Babcock-Leighton α-effect to study how helicity varies with latitude and time.     

On polarimetry in solar active regions

A new modulation procedure for Zeeman polarimeters is described and tested. The azimuth rotation by means of two steady /4-plates, combined with the common EOLM, has several advantages as compared to two-EOLM-polarimeters. The new polarimeter operates with two /4-plates which are alternately passed through the beam in front of the EOLM by means of an electro-mechanical chopper. The exact time of the /4-plate change is monitored by a photoelectric sensor. The obtained signals drive a number of relays by use of an intervening bistable electronic device. These relays allow to cut-off the erroneous Doppler signal mode and they furthermore distribute the U and Q signals into the corresponding lock-in amplifiers. As a first application of the new polarimeter, the linear polarization is measured in a sunspot penumbra. The telescope was first compensated for instrumental linear polarization down to 5 × 10^-4 by means of a tilted glass plate and well as for phase retardation down to 1° by means of a Bowen compensator.     

On polarimetry in solar active regions

The usefulness of magnetically sensitive iron lines Fe 5250.2, 6173.3 and 6302.3 for solar polarimetry is investigated. The line-to-continuum absorption coefficient ₀ for Fe 5250.2 depends strongly on temperature variations. Thus a photospheric calibration of polarimeter signals cannot be used for the different parts of an active region. This is also true for the Doppler calibration of longitudinal magnetographs.Fe 6302.5 is shown to be useful for polarimetry in active regions. A Milne-Eddington approximation is possible so that Unno's formulae are sufficient for the interpretation of polarimetric data.The ambiguity of the azimuth of the linear polarization prevents the determination of the true field structure with respect to the solar surface. The magnetic flux cannot be determined outside the disc centre without making assumptions for the unknown field structure. This determination is possible only for single stable sunspots; for irregular active regions the field configuration remains ambiguous even at the disc centre.     

Ephemeral active regions during solar minimum

Ephemeral active regions (ER) identified on Kitt Peak daily full-disk magnetograms from April through November 1975 were analyzed and compared with larger active regions during the same interval. The 1975 ER were also compared with ER data from 1970, 1973, 1976, and 1977. ER were found to vary approximately with the sunspot cycle. However, a minimum in the number of ER occurred at least one year prior to sunspot minimum. All evidence to date indicates that the early ER minimum was due to the rise of solar cycle 21 primarily in the form of ER. ER were statistically identified as belonging to both outgoing solar cycle 20 and incoming cycle 21 by maxima in their distribution in latitude and by their statistically dominant orientation as a function of latitude. From the identification of ER with specific solar cycles and the persistent presence of high latitude ER maxima since 1970, it is suggested that the outgoing and incoming solar cycles may co-exist on the sun longer than the 0–3 year period of overlap between successive cycles already known from the properties of large sunspot-producing active regions.Presently associated with Solar Physics Research Corporation, Tucson, Arizona and Visiting Astronomer at Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On polarimetry in solar active regions

The miscentering by the Doppler compensator of the Locarno polarimeter is investigated in detail. It is shown that the linear polarization is strongly falsified by this effect which also occurs at the Crimean and Izmiran polarimeters.The new design for the exit slits of the Locarno polarimeter is described. It avoids the ambiguities in the determination of the magnetic field vector that always occur when using two exit slits.A new simple electronic setup avoids most of the difficulties which are usually involved in eliminating instrumental polarization and compensating intensity fluctuations.The observational techniques for solar polarimetry at the Locarno observatory are described.To avoid mutual influences of V and U, the line centre ₀ (corresponding to V = 0 and U = max.) must coincide with the centre of slit II. Only in this case we have 234-01     

On polarimetry in solar active regions

Measurements of the circular polarization V in different lines show that the deduced magnetic field strength and flux are systematically influenced by variations of the line absorption coefficient from photosphere to spot and faculae.Disbalances between preceding and following flux seem to be due mainly to such variations rather than to real physical conditions in active regions.The spatial distribution of the normal component of the magnetic field in an active region near the disc center have been observed during two days using the temperature insensitive line Fe 6302.5. The initial field structure seems to become more and more bipolar. The increase of the flux exceeds that of the area thus suggesting the appearance of new magnetic fields. Backward extrapolation in time leads to a date of first appearance of the magnetic field which agrees with the appearance of first H anomalies.     

On polarimetry in solar active regions

Phase retardation originating in a telescope is measured by means of polarizer and analyzer. The amount of this retardation depends only on the declination of the telescope. The retardation axes rotate with the Coudé image.The influence of this retardation on a Zeeman triplet, is measured photoelectrically. Consequences for solar polarimetry are discussed: Strong effects originate from the miscentering of the asymmetric Zeeman triplet introduced by the Doppler compensator. This yields a non-vanishing V-Stokes parameter in the central exit slit and thus the often observed crosstalk between the U and V-Stokes parameters.     

On polarimetry in solar active regions

High resolved magnetograms ( 3) were obtained 3 hrs before and 1 hr after a 1b flare, respectively, the only bright flare reported for that active region. Careful comparison between both magnetograms shows that the line-of-sight component of the active region magnetic field remains constant. In particular there is no simplification of the rather complicated field structure in connection with the flare. Magnetic flux and field gradients also do not show any variation above the 3 scale. Essential changes, however, were observed after 19 hrs without flare activity. This indicates that evolutionary field changes predominate over flare related variations.     

Current convection in solar active regions

Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations.     

Bidimensional analysis of solar active regions and flares

We describe a method of solar bidimensional spectroscopy exploiting the performances of a Universal Birefringent Filter (UBF) like that of the Sacramento Peak Observatory, which enable an estimate of the profile of some chromospheric lines with moderate spectral resolution ( \((\lambda /\Delta \lambda = 2.5 x 10^4 )\) ). The numerical inversion technique of Backus and Gilbert has been used to retrieve the estimated line profiles; the capabilities of the proposed method is fully analyzed with some numerical tests and examples. Correction procedures for errors in the positions of the UBF passband, random fluctuations of the exposure times and non-uniform brightness distribution on the filtergrams are also presented. The whole method has been tested on the recovery of quiet atmosphere line profiles and the results derived for the Na D₂ line show that the proposed method is completely suitable for many investigations in solar physics.     

Further X-ray spectra of solar active regions

Measurements of the solar X-ray spectrum between 3 Å and 15 Å are reported. They were made with two slitless Bragg crystal spectrometers flown on a Sun-pointed Skylark rocket on 8 August, 1967.The use of a beryl crystal has provided higher spectral resolution than hitherto in the spectral range 12 Å and 15 Å leading, in particular, to a revised identification of the strong line at 13.71 Å. Separate components of the stronger emission lines are clearly seen from each of three coronal active regions which may be identified on radio and X-ray spectroheliograms.The absolute line fluxes are used to determine a model for each active region in terms of the differential emission measure as a function of electron temperature. Emission lines due to the transition 1s ^{2 1} S ₀ – 1s2s ³ S ₁ in several helium-like ions are identified and values of the local electron density derived from measurement of the line flux in these ions.     

The XUV structure of solar active regions

XUV spectroheliograms of 2 active regions are studied. The images are due to lines emitted at temperatures between 8 x 10⁴ K and 2 x 10⁶ K and thus are indicative of transition region and coronal structures. The hot coronal lines are formed solely in loop structures which connect regions of opposite photospheric magnetic polarity but are not observed over sunspots. Transition region lines are emitted in plages overlying regions of intense photospheric magnetic field and in loops or loop-segments connecting such regions. The hot coronal loops are supported hydrostatically while only some of the transition zone loops are. The coronal and transition zone loops are distinctly separated and are not coaxial. A comparison of direct measurements of electron densities using density sensitive line ratios with indirect measurements using emission measures and path lengths shows the existence of fine structures of less than a second of arc in transition region loops. From a similar analysis, hot coronal loops do not have any fine structure below about 2 seconds of arc.     

An evolutionary feature of solar active regions

The result that there is a time-lag between Type-I radio bursts and associated flares given in a previous paper [1] is confirmed by new statistical material.     

Magnetic and microwave structure in solar active regions

The structure of six active regions observed at 2.8 cm with the Stanford interferometer is compared with the configuration of the underlying photospheric magnetic fields, as given by the Kitt Peak magnetograph.The similar resolution and accuracy on the measured positions of both instruments allowed us to establish a more detailed spatial correspondence between radio and magnetic features than previously reached.The radio features which correspond to the cores of the active sources are always found to overlay regions of enhanced magnetic fields. Different spatial associations have been found depending on the brightness temperature of the sources. The possibility that this effect might be due to the development of the active region is also considered.     

Microflares and hot component in solar active regions

Open-shutter RHESSI observations of 3–15 keV X-rays are found to exhibit active-region transient brightenings and microflares at a rate of a least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal component fitted by temperatures of 6–14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases, and has an average duration of 131(±103) s at 7–8 keV. The duration increases with decreasing photon energy. The peak count rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases, whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating the evolution. In some of the bigger events, a second component was found in the 11–14 keV range extending down to 8 keV in some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their relevance for coronal heating. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022496515506     

A solar radio spectrograph with high time resolution

Instrumentation for obtaining high time resolution dynamic spectra of solar radio bursts at decimetric wavelengths is described. The spectrograph sweeps the frequency range of 565–1000 MHz at a rate of 100 times per second. All data are recorded both on film and as an analog signal on magnetic tape. The frequency and flux calibrations are discussed. A sampling system which allows the activity at three discrete frequencies to be plotted on a chart recorder is described.     

Cyclotron lines in the spectra of solar flares and solar active regions

It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 ± 4) G along the magnetic tube axis. The linewidth f/f 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 × 10⁶ K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.     

Simulations of magnetic-flux transport in solar active regions

We simulate the evolution of several observed solar active regions by solving a transport equation for magnetic flux at the photosphere. The rates of rotation, meridional flow, and diffusion of the flux are determined self-consistently in the calculations. Our findings are in good quantitative agreement with previous measures of the rotation rate and diffusion constant associated with photospheric magnetic fields. Although our meridional velocities are consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in our velocity determinations make this result inconclusive.Laboratory for Computational Physics.E. O. Hulburt Center for Space Research.